1. Field of the Invention
The present invention relates to metal cords, such as those used for reinforcing tires and to a process for the manufacture of the metal cords.
2. Description of the Prior Art
Metal cords, including those used for reinforcing tires, are generally manufactured in two steps: the first step is the stranding and consists, starting from single wires, in twisting the said wires together in a given direction, and the second step is the strand-laying which consists in twisting the strands together, or twisting one strand together with single wires, in the same direction or in the opposite direction to the twisting used in stranding, but, in general, to a different extent.
This sequence of operations is long and results in relatively high manufacturing costs.
A process for the manufacture of a metal wire cord is also known from Addition No. 88,666 to French Pat. No. 1,340,702, which process consists in passing the wires of two or more groups of wires, which groups are each composed of two to six wires inclusive, wound on a single bobbin, from their respective bobbins through a twisting apparatus which forms a cord from the wires, and then onto a receiving device. This gives a single-strand cord in which all the wires are twisted in the same direction and to the same extent. A cord of this type can be used in tire reinforcements. In the process, the use of a rotating twister-collector is envisaged, which is located upstream of the twisting apparatus and the function of which is to draw off the appropriate lengths of wire so as to obtain the arrangement which the wires must have in the finished cord. An over-twisting device is also provided in order to give a temporary over-twist by means of false twist, which makes it possible to set the true twist and to obtain an inert cord.
This process makes it possible to dispense with the stranding operation. However, it exhibits certain disadvantages. Since the feed comes from groups of wires, each group being wound on a bobbin, it is first necessary to make up coils of grouped wires and this constitutes an additional operation involving grouping onto a bobbin. However, this grouping operation can be avoided by collecting a group of wires directly at the outlet of a multiple wire-drawing device; however, the field of application of the process is then restricted and dependent on the use of multiple wire-drawing devices, this use being far from widespread. Furthermore, the wires of each group are drawn off simultaneously and unwind at the same speed.
Now if the finished cord is to be compact, the wires must not have the same length in accordance with their distance from the axis, that is to say in accordance with the successive layers to which they belong. In the patent in question, the simultaneous unwinding means that the wires have the same length. This results in non-uniformities along the cord. In an attempt to limit these defects, positioning means (positioning guides and perforated plates), located between the feed bobbins and the twisting apparatus, are provided. However, apart from making the device more complex, these means do not succeed in eliminating the nonuniformity defects.
This process produces cords of which the properties, in particular the tensile strength, can vary along the cord because of the lack of uniformity in positioning the strands.
Now, metal cords for reinforcing tires must possess both a high tensile strength and a good fatigue resistance, coupled with the smallest possible diameter. In fact, in the making of calendered plies, the diameter of the cord determines the thickness of the ply. A thin ply has a dual advantage: from the economic point of view the amount of rubber used is smaller and the cost of the ply is reduced, and, from the technical point of view, a thinner ply provides a better resistance to heat.
An attempt has been made to reconcile these contradictory characteristics in specific manufactured products.
Thus, U.S. Pat. No. 3,358,435 described a cord of the 3+6 type, that is to say with 3 core wires and 6 outer wires, which exhibits improved compactness. The 6 wires of the outer layer are laid around the core in the same direction and with the same pitch as the latter; however, in order to make the cord compact, these 6 wires consist of two groups of three wires, the diameter of the wires being different in the two groups and different from the diameter of the core wires. In this manner, while retaining an identical external diameter to that of the conventional 3+6 cord, 3 wires of larger diameter are used for the outer layer and this results in a better packing and a higher tensile strength. However, the improvement in the tensile strength only results from the increase in the linear mass, which constitutes a handicap in the tire. Moreover, this method of construction is restricted to the 3+6 structure. Furthermore, it necessitates the use of cores of different diameters, which results in a deformed cord, it is expensive, it carries the risk of mixing the wires together, and, in particular, it requires a device for the very strict positioning of the strands, any transposition of two outer wires in the process leading to a poor result.